NOTES
2359
--
Table I1 : Equilibrium D a t a
+
0.5K1
t ia
0 0031 i: 0 0026
1 19(-)
(l.6Kz 0.5Ka)
tab
0 0342 & 0 0095
3.62(++)
fC
7
a Students’ t-test for significance of intercept as compared to zero. Students’ t-test for significance of slope as compared to
’
zero.
Degrees of statiotical freedom
(ill ). 2
fi
- 2
tive ion. Appearance potentials of CF2+ from CF3X, X = F, C1, Br, and I, have been measured by Dibeler, Reese, and Nohler‘; and a separate set of these same measurements has been listed by Craggs and ?\4asseyJ2 mho also included CF2Clz. These two lists show considerable divergence and no consistent value for AH*(CF2+)can be deduced from those few cases for which the necessary auxiliary data are available. A direct electron impact measurement of the ionization potential of the CF2 radical has been reported by Reed and S n e d d e r ~who , ~ obtained a value of 13.3 e.v. We have now measured the appearance potential of CF2+ froin two new compounds, CF3H and CFzHz. These measurements lead to a consistent value for AHf (CFz+) and demonstrate that Reed and Snedden’s value for I(CF2) is too high. Appearance potentials were measured on a Consolidated Electrodynamics Corp. Model 21-103C mass spectrometer kindly made available by Harvard University. Ionization efficiency curves for the CF2+ ions were not parallel to that of the calibration curve (argon) and the appearance potentials were obtained by conventional extrapolation of linear plots. Results are given in Table I.
the monomer, HOBO(g), is only a minor product in comparison to the trimer, (HOB0)3(g). Thermodyna,mic function is for &B03(g)> HzO(g), and B203(l) have been tabulated by the Sational Bureau of S t a x l d a r d ~ . ~The . ~ entropy of (HOBO)o(g’) has been estimated by White, et d o On the basis of these tabulated functions the value of Ka a t 1147°K. is 0.0038 atm.””’. On the basis of this value and the value of the sltope of the line illustrated in Fig. 3, the value of K z is 0.0215 3t 0.0063 atrn.-”’. Employing the entropy listed by White, et aLJ6the resulting heat of formation of (HOBO)a(g) a t 1147°K. is -544.8 0.6 kcal./mole. Interpolations within the heat capacity data tabulated for (HOB0)3(g)by White, et d,’? Table I 2.0 I.rcal./mole for the leads to a value of -545.8 heat of formation at 298°K. The uncertainty in thit; figure arises from the uncertainty in the heat capacity Compound data.
*
(4) Xational Bureau of Standards (E. S.),Circular 500, E. S.Government Printing Office, Washington, D. C., 195% ( 5 ) Sational Bureau of Standards (C. S.), Report 7093, TJ.S.Government Printing Office, Washington, D. C., 1960. ( 6 ) D. White, D. E.Mann, P.N. W’alsh, and A. Sommer, J . Chem. Phys., 32, 488 (1960).
Appearance Potentials of the Difluorornethylene Positive Ion
by W. C. Steelr: Depai tment of Chemistry, T u f t s University, Metiford, Massachusetts (Received February 84, 2984)
The continuing high level of interest in the chemistry of organic fluorine compounds has created a similar interest in the thermochemical properties of fluorocarbon radicals and ions. However, this area is characterized by a paucity of well established va,lues. This is particularly true of the difluoiromethylene posi-
CFsH CF2H2
Ap(CFn+), e.v.
A H i (CFa 9, koal. mole-’
1 4 . 7 i= 0.4 14.8 i 0 . 4
240 =k 10 234 & 10
The heat of formation of the difluoromethylene positive ion has been calculated from the appearance potentials and known heats of formation of CF3H,4 CF*H2,4and HF,6 assuming the reactions
+ e CF2+ + HF + 2e CFz+ + Hz + 2e CF2Fr2 + e --+
CFsH
---j
The resulting values (Table I) agree to well within experimental uncertainty. Added support may be found from the appearance potential data of Dibeler, (1) V. H. Dibeler, R. M . Reese, and F. L. Mohler, J . Res. iVatZ. B u r . Std., 57, 113 (1956) (2) J. D. Craggs and H. W. S. Massey in “Handbuch der Physik,” S. Flogge, Ed., Vol. XXXVII/l, Springer-Verlag, Berlin, 1959, p. 314. (3) R. I. Reed and W. Snedden, Trans. Faraday Soc., 54, 301 (1958). (4) G. A. Neugebauer and J. L. Margrave, J . P h y s . Chem., 6 2 , 1043 (1968). (5) H . 31.Feder, W. N. Hubbard, S.S.Wise, and J. L. Margrave, ibid., 67, 1148 (1963).
V o l u m e 68, Number 8
A u g u s t , 1964
2360
NOTES
Reese, and Mohler’ for CFz+ from CF, and CF,Cl, which, when combined with the required heats of formation6 and assuming the reactions
+ e --+ CFz+ + Fz + 2e CF&1 + e -+-CFz+ + F + C1 + 2e CF,
lead to values for AHf(CF2+) of 249 and 245 kcal. mole-’, respectively. I n each of the four cases other reactions can be postulated, but these lead to widely divergent results. The above results show AHf (CFz+) 5 240 & 10 kcaf. mole-’, which is an upper limit since excess energy may be present in all four dissociation processes. Appearance potential studies of CF2+ from tetrafluoroethylene’~*and the heat of formation of CzFd9 lead to the relation aHf(CF2+) AHf(CF2) 5 197 kcal. mole-’. Taken with the present result for AHf(CF2+), this gives AHf(CF2) = -43 kcal. mole-’, which is within the established limits of -18 kcal. mole-1 and -45 kcal. Furthermore, when taken with the lower limit for AHf(CF2),the present result allows calculation of an upper limit of 12.4 e.v. for I ( C F 2 ) ,which is well below the previous experimental value.3
+
(6) “Best values” from C. R. Patrick in “Advances in Fluorine Chemistry,” Vol. 11, M.Stacey, J. C. Tatlow, and A. G. Sharpe, Ed., Butterworth, Inc., Washington, D. C., 1961, p. 1. ( 7 ) J. L. Margrave, J . Chem. P h y s . , 31, 1432 (1959). ( 8 ) C. Lifshite and F. A. Long, J . P h y s . Chem., 6 7 , 2463 (1963). (9) G . A. Neugebauer and J. L. Margrave, ibid., 60, 1318 (1956). (10) J. F. Reed and B. S. Rabinowitch, ibid., 61, 598 (1957). (11) B. Atkinson, J . Chem. Soc., 2684 (1952). (12) B. A. Thrush and J. J. Zwolenik, Trans. Faraday Soc., 59, 582 (1963).
Comparison of the Photo- and RadiationSensitized cis-trans Isomerizations of Polybutadiene
by Morton A. Golub Stanford Research Institute, Menlo P a r k , California (Received February 26, 1964)
than that suggested for the isomerizing ability of the thiyl radical (-lo4) produced in the analogous radiation chemical isomerization of polybutadiene. Since the photo- and radiation-sensitized isomerizations are chain reactions with similar activation energies (3.67 and 2.3 kcal./mole, respectively) and with presumably the same type of free radical rnechanism,ll2 there is no reason for the yields in these two cases to differ by so much. Actually, the number given in the radiation study2 was only a rough estimate: it was based on assumptions made in the absence of definite information about energy transfer between benzene and diphenyl disulfide, or production of benzene triplet states. The purpose of this note is to re-examine the radiation yield with the aid of recent G‘ values for excited benzene molecule^,^-^ and to show that the thiyl radicals have, in fact, very similar isomerizing efficiencies whether produced by ultraviolet or by y-rays. This finding is significant because it adds to the small but growing literature on detailed comparisons between photo- and radiation chemistry.6 From data on the radiation-induced cis-trans isomerization of 2-butene in benzene, Cundall and Griffiths3t4 concluded that G(C6H6 triplet) was about 4.4. Moreover, 2-butene, a t concentrations greater than 0.2 M , was considered to acquire virtually all of the excitation energy deposited in benzene. If our previous data on the diphenyl disulfide sensitized isomerization of polybutadiene* are replotted (see Fig. 1) in a form like that of Cuiidall and Griffiths (Fig. 1, ref. 4), we see an analogous approach to a limiting isomerization yield (Go 1400 cis double bonds isomerized initially per 100 e.v. absorbed by the whole solution) a t [C6H6SSC&,] > 0.14 M . Consequently, the production of excited sensitizer molecules via
-
C6&*
+ CijHsSSC6Hs -+CsHe f CG&SSC~&* (I)
likewise can reach a saturation G value of about 4.4. Taking the energy deposited in the disulfide molecule to be 3.6 e.v. or 86 kcal./mole (corresponding to the lowest benzene triplet) , which is iomewhat greater than the energy imparted by 3650 A. light (78 kcal./ (1) G . R. Seely, J Am. Chem. Soc., 8 4 , 4404 (1962).
Seely’ has investigated the diphenyl disulfide sensitized cis-trans isomerization of polybttadiene in benzene and other solvents, using 3650-A. light to photolyze the disulfide. He concluded that each thiyl radical thus formed isomerized about 370 polybutadiene double bonds. This number is very much lower T h e Journal of Physical Chemistry
A. ‘I Golub, . ibid., 81, 54 (1959). (2) > (3) R. B. Cundall and P. A. Griffiths, ibid., 85, 1211 (1963). (4) R . B. Cundall and P. A. Griffiths, Discussions Faraday Soc., in press. (5) W. Ando, K. Sugimoto, and S. Oae, Bull. Chem. Soc. J a p a n , 3 6 , 893 (1963). (6) M. S. Matheson, Proc. end Intern. Conf. Peaceful Uses At. Energy, Geneva, 29, 385 (1958).
